The present invention relates to an optical pickup device, and more particularly, to an optical pickup device which can be employed for different formats of a disk recording medium.
In general, an optical pickup device records and reproduces information on and from a recording medium in a non-contact manner by being employed in a compact disk player (CDP), a digital video disk player (DVDP), a CD-ROM driver, and a DVD-ROM driver.
The optical pickup device employed by a DVDP which enables high density recording/reproducing of information is required to be compatible for the compact disk (hereinafter called CD) family, such as a CD (compact disk), a CD-R (CD-Recordable), CD-I (CD-Interactive) and a CD-G (CD-Graphical), not only a DVD (digital video disk).
The thickness of the DVD differs from that of the CD due to differing mechanical disk inclination allowances and the numerical apertures of an objective lens of the respective standards. That is, the thickness of the conventional CD is 1.2 mm, whereas that of the DVD is 0.6 mm. Since the thicknesses of the CD and DVD are different from each other, when an optical pickup device for a DVD is used to reproduce information recorded on a CD, spherical aberration due to the difference in the thicknesses occurs. Thus, a sufficient light intensity required for recording information is not obtained, or deterioration occurs in the reproduced signal. Also, the wavelength standards of the reproduced light for a DVD differs from that for a CD. That is, the wavelength of the conventional CD reproduction light is approximately 780 nm, whereas that of the DVD is approximately 650 nm.
In consideration of the above points, optical pickup devices compatible for multiple formats have been suggested. These optical pickup devices focus on overcoming such problems caused by the differences between the disk formats, and use a single light source. That is, a light source emitting light of a short wavelength of approximately 650 nm which is appropriate for a DVD format, is employed. In this case, deterioration in the reproduced signal increases about 5% relative to an optical pickup employing a light source emitting light of about 780 nm, which is within the reproduction allowance, causing little problem.
However, when a CD-R is employed as a recording medium, there is a difference in sensitivities between a light source emitting light of approximately 650 nm and a light source emitting light of approximately 780 nm, as shown in FIG. 1. Since the CD-R has an organic dye film recording layer, when using the light of 650 nm, signals are not reproduced by a low reflection ratio of a disk due to the difference in sensitivities.
To overcome such a problem, as shown in FIG. 2, another conventional optical pickup employing two light sources has been suggested. Referring to FIG. 2, a first light source 21 for a disk 10a having a relatively thin thickness, e.g., a DVD, emits light of approximately 635 nm, and a second light source 31 for a disk 10b having a relatively thick thickness, e.g., a CD, emits light of 780 nm.
The light emitted from the first light source 21 is reflected by a polarized beam splitter 23 and sequentially passes through an interference filter 33, a 1/4 phase delay plate 11 and a holographic device 13. Also, the optical path of the light emitted from the second light source 31 is converted by the interference filter 33 and then the light passes through the 1/4 phase delay plate 11 and the holographic device 13. A predetermined diffraction pattern 13a is formed at a central portion of the holographic device 13. Since the light emitted from the first light source 21 is input with a radius greater than that of the diffraction pattern 13a, the light becomes a O-th order diffracted light which is not diffracted while passing through the holographic device 13 and is focused on the relatively thin disk 10a. Meanwhile, the light emitted from the second light source 31 is diffracted by the diffraction pattern 13a of the holographic device 13 to a predetermined degree to be a +1st order diffracted light so that the light is focused on the relatively thick disk 10b. The light after passing through the holographic device 13 is focused on the disk 10 by an objective lens 15.
Such an optical pickup device can be used when a CD-R is employed as a recording medium. However, it is difficult to assemble and arrange the holographic device 13 in which the diffraction pattern 13a is formed and the optical efficiency of the +1st order diffracted light diffracted by the holographic device is drastically reduced by 34% with respect to a case in which the holographic device is not employed.